During the final years of the fur trade, Bridger, with partner Louis Vasquez, planned and constructed what was to be Fort Bridger, located on Black's Fork of the Green River. This new enterprise was to become one of the principal trading posts for the western migration, established specifically to serve the wagon trains heading to the far West. Bridger's post served many immigrants heading west, including the ill-fated Donner-Reed party, which followed the Hastings Cutoff through Utah, then across Nevada to California.
In June 1847 Bridger had his first encounter with the Mormon pioneers near the mouth of the Little Sandy River. At this gathering, Bridger and Brigham Young discussed the merits of settling in the Salt Lake Valley. Also during this meeting Bridger drew his map on the ground for Young depicting the region with great accuracy and conveyed to the Mormon leader his misgivings regarding the agricultural productivity of the Salt Lake area. This first meeting between the Mormons and Bridger appears to have been pleasant, yet this relationship was to become a bittersweet one for Bridger.
The coming of the Mormons increased the number of immigrants at the fort. However, the Mormon settlements attracted away a significant portion of Bridger's trade, including that of the Indians, causing economic hardships for the post.